UHF RFID tags and labels are often designed to meet customer requirements in term of mechanical strength, thermic resistance and read range performance.
Low cost, popular UHF RFID tags for industry and logistics applications comprise, in general a UHF inlay packaged in a housing (generally made of plastic) containing the inlay in order to protect the fragile antenna and chip from outdoor conditions: rain, impact shocks, snow, dust etc . . . .
As example, a UHF inlay could comprises a 6-10 μm thick aluminium etched antenna on a 50-100 μm thick substrate made of PET, on which a RFID chip is soldered. The inlay is thin and therefore flexible. The inlay has a thin layer of adhesive on the bottom side, so one can stick the inlay thanks to its adhesive layer (just like stickers) to a substrate for example.
A housing is generally formed by a bottom part and a cover which are sealed together once the RFID inlay has been placed inside. In some other embodiment, the housing could be provided as one single piece of material which is injected around the inlay.
Industry and logistics applications often require the tag to be attached/affixed on metallic items/objects. For UHF, this is bad news since at UHF frequencies the reflected waves from the metal is added to incident waves in a destructive way, and therefore the UHF antenna inside the tag collects a very weak signal/very small energy to feed the UHF chip. A popular technique to solve this issue is to use some specific antenna types rather than others; typically antenna types that are able to collect more energy than others for the same form factor. A popular antenna type is the PIFA antenna, well-known in the GSM world, and now becoming the antenna of choice in the RFID UHF world.
The PIFA antenna of course must be adapted to RFID applications so that visually it is different than the ones used in the GSM world, but its principle is the same. This antenna requires a substrate since the PIFA antenna is a “3D” antenna (a PIFA antenna must have 2 conductive layers in parallel, connected together by a third perpendicular conductive surface layer). For the low cost UHF RFID tags used in industry and logistics applications, the substrate is in general made of plastic with a thickness ranging from 1 to 15 mm.
The RFID tags described above are typical low cost UHF RFID tags for industry and logistics applications.
Another example of an RIFD tag or label is given in US 2005/0093677 to Forster et al., in the following description referred to as “US'677”.
Specifically US'677 discloses an RFID device comprising a hybrid loop-slot antenna which increases its readability. To this effect, the radio frequency identification (RFID) device comprises a conductive antenna structure having an elongated slot therein. Parts of the antenna structure on both sides of one end of the elongated slot are coupled to a wireless communication device, such as a RFID chip or interposer. On the opposite end of the elongated slot, parts of the antenna structure at both sides of the elongated slot are electrically coupled together, for instance by a conductive part of the antenna structure. Typically, all parts of the antenna structure may be parts of a continuous unitary layer of conductive material deposited on a substrate. The antenna structure with the elongated slot therein is intended to increase the readability of the RFID device, particularly in directions out from the edges of the RFID device. The antenna structure may be directly conductively coupled to the wireless communication device. Alternatively, the antenna structure may be indirectly (reactively) coupled to the RFID device, such as by a capacitive coupling.
On the other hand, there is also a need in the market to provide tunable or fine-tunable antennas for RFID tags that perform well in metallic environments.
Many antenna types exist (example: Dipole antenna, patch antenna such PIFA antenna, horn antenna etc.) for all kinds of applications as is illustrated for example in US'677 discussed above: as shown in this publication, the RIFD tags may be used to mark clothing items or bottles or bottle caps.
A known antenna that is suitable for such need of good performance in a metallic environment is the slotted type such the one described in the book of John D. Kraus “ANTENNAS”, third edition (pages 304-320), which has been adapted to the RFID because of the reasons mentioned hereunder.
The antenna described in this book is for typical applications such as radar, satellite, GSM, microwave systems etc. In such typical applications, the impedance of reference is the standard impedance of 50 Ohms, which is obtained using adaptive/matching circuits between antenna and source. But in the more recent domain of the RFID UHF world, because one of the most important requirement/constraint is the low cost of the UHF tag, one has to avoid the use of any additional circuit except for the transceiver chip and antenna.
Furthermore, an adaptive circuit dedicated to impedance matching will create energy losses and since UHF RFID tags are passive tags they only use the power from a radio wave source. Therefore any energy loss will strongly reduce the sensitivity performance of the chip.
In other words, the antenna should be able to impedance match itself to the transceiver chip impedance for maximum power transfer without the use of an additional circuit and one cannot directly use the antenna design proposed by Kraus.
It is therefore an aim of the present invention to provide an RFID tag that may be easily tuned to take account of its desired application and environment.
In addition, it is a further aim of the present invention to provide an RFID tag that may easily be fine-tuned in a passive manner.
Moreover, RFID tags have become more popular today and they are used in many different applications for example for marking all kinds of products in everyday life, as illustrated in the US'677 mentioned above as an example.
In this extend, there is a need to provide patch antennas for UHF RFID tags that may easily be tuned to simplify the production processes and deliver products that may easily be adapted at the customer's end to the desired properties. Such customization is interesting as it allows the fabrication of standardized products that may be easily adapted to the need of the end user after production, rather than the fabrication of many different products each individually customized at the early production stage.
A mass production of standard RFID tags also allows a drastic reduction of cost for the end user who can then carry out the necessary customization that suits his needs. A good example of it would be a unique UHF tag design which would only need a minor “personalization” to be operational in the different geographical markets: either Europe, USA and Japan, as each market has specific operational frequency (Europe: 865 MHz, USA: 902-928 MHz, Japan: 956-960 MHz).
In addition, it is known that such prior art slotted UHF RFID tags have temperature limitations and they usually only withstand a maximum temperature of 65° C. according to storage temperature tests that have been carried out.
Specifically, one has noticed that above 65° C., electrical measurements of tags showed an unacceptable detuning of the tag: concretely, above this temperature the resonance frequency of the tag shifted away from the frequency of interest. After a careful analysis, it was observed that the tags which were subjected to these temperatures accumulated gas underneath their inlay and the gas was in fact generated by the adhesive used to glue the inlay to the substrate
The gas generated between the substrate and the inlay at these temperatures, by accumulation, then created a pressure increase between the substrate and the inlay which then caused a partial separation of the layers. This separation process had the effect of creating a de-tuning (shift in resonance frequency) of the tag because the dielectric constant of the immediate surroundings of the antenna was different after the partial separation.
In normal tag design conditions, this de-tuning does not create a problem, but tags in the presently considered field have a particularity: they have a very high Q-factor when affixed on metallic items, because their antenna has been designed to perform well very near metallic objects. Because the Q-factor is high, any slight de-tuning has the consequence of rendering the tag practically unusable. Thus, it is necessary to maintain as much as possible the precise resonance frequency of the tag avoiding any de-tuning, even in extreme conditions such as high temperature, otherwise the tags will not perform as expected in the considered environment.
This temperature limitation has been regarded as too low in the field, and there has been a constant need to find a solution improving this aspect and thus to design tags that are able to resist to higher temperatures with no performance degradation, especially in the applications mentioned hereabove.
In view of the above discussion, there is a clear need to develop an improved version of tags for RFID UHF applications that does not possess the identified drawbacks of the known devices of the prior art.